perf/x86/rapl: Remove the unused get_rapl_pmu_cpumask() function

commit 9e9af8bb ("perf/x86/rapl: Clean up cpumask and hotplug")
removes the cpumask handling from rapl. Post that, we no longer need the
get_rapl_pmu_cpumask() function. So remove it.

Signed-off-by: Dhananjay Ugwekar <Dhananjay.Ugwekar@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Zhang Rui <rui.zhang@intel.com>
Tested-by: Zhang Rui <rui.zhang@intel.com>
Link: https://lore.kernel.org/r/20241115060805.447565-2-Dhananjay.Ugwekar@amd.com